ALFRED PASIEKA / SCIENCE PHOTO LIBRARY ALFRED PASIEKA / SCIENCE PHOTO LIBRARY
Irregular heartbeat. Computer artwork of an electrocardiogram (ECG) showing two irregular heartbeats (top and middle) and a normal heartbeat (bottom). An ECG measures the electrical activity of the heart. Contractions are caused by electrical signals between the upper (atrial) and lower (ventricular) chambers. The tall peak indicates the pumping action of the ventricles.
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